Paper sizing process



United States Patent() This invention relates to compositions for sizing and coatmg the surface of paper and paperboard, and more particularly to novel compositions and processes including polyvinyl alcohol and alkali metal alginates.

A widely used surface sizing agent for paper and paperboard is polyvinyl alcohol. Polyvinyl alcohol ,is seldom used alone for surface sizing. When used alone, excessive amounts must be applied because of the tendency to penetrate into the sheet. Since only the polyvinyl alcohol remaining on the surface is functional, this mustbemaintamed by increasing the amount applied or adding one or more other mgredients to the size solution to increase viscosity, or give body, to the solution. Cooked starch is widely used for this purpose. Clay, or othermineral pigments, are sometimes added to size formulations to further improve surface characteristics. .When this isthe case the need for penetration controlis even more critical since polyvinyl alcohol 'must also serveas a binding agent for the pigment particles. Insufiicient binding results in dusting and poor. surface printing properties.

Another method sometimes used to hold polyvinyl alcohol on the surface is to pretreat the surface with borax solution. The reaction .of polyvinyl alcohol 'and borax produces a gel-like material which tends to remain at the surface. There are several disadvantages to the borax pretreatment technique. First, it is' a two stage process. Secondly, the polyvinyl alcohol solution is often contaminated by leaching of borax from the sheet. Third, borax treatment is capable of harming the internal sizing of the sheet.

An' object of the present invention is to provide a composition and process for paper sizing which permits the utilization of polyvinyl alcohol with improved results and lessened consumption of this materiaL.

Another object of the invention is to providea composition and process for paper sizing utilizingp'olyvinyl alcohol and alkali metal alginates in combination.

Another object of the invention is to provide a paper sizing composition and method which permits the utilization of polyvinyl alcohol without the attendant disadvantages described hereinabove.

Other objects of the invention will become apparent as the description thereof proceeds.

The sizing, which is here considered synonymous with surface coating, of paper is of broad application, and includes paper of all types and paperboard, chip board, card stock, and the like. The purpose may be merely to give a suitable finish, or to permit subsequent printing, embossing, and the like. The methods involved are well known and conventional. For example, on light weight paper grades, the size is applied to the surface from an aqueous solution by means of a size press or size tube located in the drying section of the paper machine. On paperboard, the application may be made at the size press and/or from one or more water boxes at the calender stack. On cylinder machines, pigmented size solutions are sometimes applied from a coating station located in the drying section.

The polyvinyl alcohol of commerce is made by the hydrolysis of polyvinyl acetate. As a practical matter the hydrolysis is seldom carried to completion, so that a minor proportion of acetate groups may remain in the final product. Consequently, by the term polyvinyl alcohol as used in this disclosure and in the claims which follow,

r 3,298,862 Patented Jan. 17, 1967 we mean to include not only completely hydrolyzed polyvinyl acetate but also those grades which have been hydrolyzed to the extent of at least 70%. Products of this type are water dispersible, and are readily available from a variety of manufacturers.

As is well'known, 'alginates are derived from certain species of seaweeds, and are widely available articles of commerce. In our invention, we use water soluble alginates, which term includes sodium alginate, potassium alginate, ammonium alginate, and the like. It may be noted, however, that from-the practical standpoints of efiicacy, availability, and price, We find sodium alginate to be best. Any of the commercially available viscosity grades may be used.

As has been mentioned, the employment of sizing compositions in this art is to change and improve the surface characteristics of the paper,- and to this end various additives in addition to film forming agents are often "included in the sizing compositions. These include very frequently pigments, such as clay, precipitated chalk,

blanc fixe, titanium,dioxide, and the'like; coloring materials of various types, including-dyes, lakes, and the like; oils .andwaxes' toaid in obtaining a high finish; anti-foamihgagents to prevent frothing of the mixture, especially with certain types of application equipment; and like minor additives. It will therefore be understood that our novel..sizing compositions may include total sizing.composition. This sizing mixture is applied in any suitable fashion,'including the conventional method referred to above, to ,the paper-material, and this is followed by drying of the so-treated paper. When desired, other treatments, which may be conventional, may be applied, such as calendering, printing, overcoating with other types of coating compositions, and the like.

As mentioned, we have found that combinations of polyvinyl alcohol and a water soluble alginate such as sodium alginate when used in accordance with the invention markedly reduce the tendency of the sizing solution to penetrate the paper. Moreover, compositions in accordance with our invention are found to have solution viscosities higher than predicted from the viscosities of the individual components. Furthermore, the penetration of the solutions into paper is retarded very much more for combinations in accordance with the invention than for solutions of the individual substances. Striking evidence of such synergistic effect will be shown in a table of results to follow later herein.

In the tests, the results of which are given in the table as follows, percentages as noted of a commercial polyvinyl alcohol (PVA) and of a commercial sodium alginate (SA) were added to water, and the viscosity of the resulting solutions determined .with a Brookfield viscometer. The viscosities found are given in centipoises (cps) in the third column of the table. Penetration rates were measured in the laboratory penctrometer. The instrument is a modification of that described in The Water Retention Test in Evaluating Coating Color by J. C. Stinchfield, R. A. Clift, and J. I Thomas (TAPPI, 41, 77-79 (1958)). The test consists of measuring under controlled conditions the time re- 3 quired for an aqueous solution to penetrate through standard blotter paper of 0.020 inch thickness. A conductivity cell is used to measure the end point. The penetration times in seconds are given inthe last column of the table.

TABLE I Composition,

Percent Brookfield Vis- Penetration cosity, 72 F., 60 Time, sec.

r.p.m., cps. PVA SA It is readily apparent that the time required for a polyvinyl alcohol solution to penetrate i the standard paper can be dramatically increased by the addition of an alginate in accordance with our invention. As a result, the application of this invention to paper sizing will result in a more efficient utilization of polyvinyl alcohol permitting a reduction in the amount applied 'to the Further benefit-s are obtainedby the use of this invention for the surface sizing of paper since the alginate is, in itself, an eflicient sizing agent for improving-such surface properties as oil resistance, porosity, smoothness and printabilit-y.

We have found that the benefits of our invention-are likewise obtained when the other additives noted here; inabove, including such materials as clays, coloring materials, and the like, have been included in the sizing composition; and likewise we have found that other film forming agents or hydrophilic colloids such as gelatinized starch may be included as well.

A commercial polyvinyl alcohol marketed under the trade name Elvanol, and of commercial grade .71-24,

characterized by having a degree of hydrolysis Within the range of 97.9 to -98.'7% and a viscosity 'in' a 4% aqueous solution at 20 C. of about 23 to "28 centipoises, was used together with a commercial sodium alginate of medium viscosity derived from Pacific Coast kelp, and sold under the trade name Kelgin. A mixture of these two materials was made up in water, the concentrations of polyvinyl alcohol and sodium alginate being respectively 1% and /2%. This was used to size paperboard stock of .050 inch thickness. It was made up of groundwood and 50% sulfite pulp. Excellent surface retention was noted upon coating, and after subsequent drying of the so-sized paperboard, a very good smooth. finish was obtained. 1

It will be understood that while our invention has been described in terms of and with the aid of various illustrative examples, numerous changes in details, proportions, ingredients, and the like may be made within position contains starch.

' 4. The "process of claim 3 wherein said aqueous composition contains a pigment.

References Cited by the Examiner UNITED STATES PATENTS 2,256,853 9/1941 Schwartz 260--17.4 XR 2,290,633 7/1942 Cate 26017.4 2,364,738 12/1944 Marberg et al. 26017.4 2,456,295 12/1948 Mast 26017.4 XR 3,091,509 5/1963 Suda et al. 26017.4

OTHER REFERENCES Alexander, Colloid Chemistry, vol. VI, 1946, Reinhold Publishing Company, New York, p. 721 relied on.

WILLIAM H. SHORT, Primary Examiner.

MILTON STERMAN, Examiner.

S. N. RICE, J. NORRIS, Assistant Examiners. 

1. THE PROCESS OF SIZING PAPER WHICH COMPRISES THE STEPS OF: APPLYING TO A SURFACE OF SAID PAPER AN AQUEOUS COMPOSITION CONSISTING ESSENTIALLY OF WATER, FROM 1/2% TO 10% POLYVINYL ALCOHOL BY WEIGHT OF SAID COMPOSITION, AND FROM 1/10% TO 1% OF A WATER SOLUBLE ALGINATE BY WEIGHT OF SAID COMPOSITION; AND THEREAFTER DRYING SAID PAPER. 